Arsenic often is present as a contaminant in mineral acids, for example, phosphoric acid and hydrochloric acid, and is desirably removed therefrom to permit the acid to be used in a variety of applications. For example, thermal grade phosphoric acid is contaminated with dissolved arsenic at a typical level of 20 to 50 ppm.
One manner of processing such phosphoric acid to remove the arsenic values is to contact the phosphoric acid, while hot, with sodium chloride, which forms phosphate salts from the phosphoric acid and HCl and AsCl.sub.3 gases. The gases are removed and contacted with water to form hydrochloric acid containing the arsenic trichloride. The hydrochloric acid then is dearsinated by precipitation of the arsenic values as As.sub.2 S.sub.3 by the addition of Na.sub.2 S or H.sub.2 S. The purified hydrochloric acid then may be used for a variety of purposes.
This prior art process is effective in removing the arsenic contamination but a large excess of sulfide ion is needed when the HCl concentration is higher than about 30% or the temperature is elevated and the remaining sulfide ion in the treated acid must be destroyed.